Omit compilation of cxa_thread_atexit.cpp on Linux

That file pulls in a symbol that's only present on glibc 2.18.  We
don't want to increase the glibc version on Linux, so compile without
it.  It only provided one symbol that was unused in chrome, anyway.

BUG=749077
R=thakis@chromium.org

Change-Id: I977509eefbff639e3b428af634355591f2058332
1 file changed